Add UI-disabled tests for UserController

Added dispatchUserSwitchComplete and failedStartUserInForeground tests
to make sure that WM.setSwitchingUser is called appropriately.

Also set the default for UserSwitchUIEnabled to true.

Bug: 30751014
Change-Id: I9143178cb5da49c0c9524560e9fca2ab6f0d2b00